Physiological monitoring systems and apparatus, which are adapted to acquire signals reflecting physiological characteristics, are well known in the art. The physiological characteristics include, for example, heart rate, blood pressure, blood gas saturation (e.g., oxygen saturation) and respiration rate.
The signals acquired by the noted physiological monitoring systems and apparatus are however composite signals, comprising a desired signal portion that directly reflects the physiological process that is being monitored and an undesirable signal portion, typically referred to as interference or noise. The undesirable signal portions often originate from both AC and DC sources. The DC component, which is easily removed, results from the transmission of energy through differing media that are of relatively constant thickness within the body (e.g., bone, tissue, skin, blood, etc.).
Undesirable AC components of the acquired signal correspond to variable or erratic noise and interference, and thus have been conventionally quite difficult to characterize and remove.
One example of a physiological monitoring apparatus, wherein the measured signal can, and in many instances will, include undesirable signal components, is a pulse oximeter.
Pulse oximeters typically measure and display various blood constituents and blood flow characteristics including, but not limited to, blood oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the flesh and the rate of blood pulsations corresponding to each heartbeat of the patient. Illustrative are the apparatus described in U.S. Pat. Nos. 5,193,543; 5,448,991; 4,407,290; and 3,704,706.
As is well known in the art, a pulse oximeter passes light through human or animal body tissue where blood perfuses the tissue, such as a finger, an ear, the nasal septum or the scalp, and photoelectrically senses the absorption of light in the tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
Two lights having discrete frequencies in the range of about 650–670 nanometers in the red range and about 800–1000 nanometers in the infrared range are typically passed through the tissue. The light is absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light passed through the tissue varies in accordance with the changing amount of blood constituent in the tissue and the related light absorption.
The output signal from the pulse oximeter, which is sensitive to the arterial blood flow, contains a component that is a waveform representative of the patient's blood gas saturation. This component is referred to as a “plethysmographic wave or waveform” (see curve P in FIG. 1).
The plethysmograph signal (and the optically derived pulse rate) may however be subject to irregular variants that interfere with the detection of the blood constituents. The noise, interference and other artifacts can, and in many instances will, cause spurious pulses that are similar to pulses caused by arterial blood flow. These spurious pulses, in turn, may cause the oximeter to process the artifact waveform and provide erroneous data.
Several signal processing methods (and apparatus) have been employed to reduce the effects of undesirable signal components on the measured signal and, hence, the derived plethysmograph waveform. Illustrative are the methods and apparatus disclosed in U.S. Pat. No. 4,934,372, which correlate a subject's electrocardiogram waveform with the acquired signal to identify desired portions of the signal to more accurately detect blood constituents.
Similarly, U.S. Pat. Nos. 5,490,505, 6,036,642, 6,206,830, and 6,263,222, all disclose signal processors that generate either a noise reference or a signal reference which is used to drive a correlation canceler and generate a waveform that approximates either the desired or undesired component of the acquired signal. A primary intended application of the noted signal processors is the measurement of blood oxygen saturation in a manner that minimizes the effect of motion artifacts. However, a consequence of the process used to generate the reference is that a third optical signal must be acquired to provide ratiometric calculation of saturation.
Accordingly, each of the noted prior art references require the acquisition of additional signals to help measure blood oxygen saturation. As such, these systems are inherently more complex and costly. Further, the noted references are primarily concerned with filtering out motion artifacts. Therefore, these references are not tailored to the removal of undesired signal components that arise from other sources.
It is therefore an object of the present invention to provide a cost effective, reliable means of determining a physiological characteristic by detecting a minimum number of signals.
It is another object of the invention to provide a method for processing signals reflecting a physiological characteristic that does not require correlation canceling.
Another object of the invention is to provide a method for processing signals reflecting a physiological characteristic that minimizes undesirable signal components.
It is yet another object of the invention to provide a method and apparatus for correcting signals reflecting a physiological characteristic that does not require a pulse waveform model or data from preceding pulse waveforms.
Yet another object of the invention is to provide a method and apparatus for correcting signals reflecting a physiological characteristic using data from a single pulse.
A further object of the invention is to provide a method and apparatus for determining arterial oxygen saturation with improved accuracy.
It is another object of the invention to provide a method for processing oximetry signals based on specific time dependent differences during a single pulse.
Another object of the invention is to provide a method for improving an oximetry signal based on analytical, mathematical steps that are analytically transparent, interpretable and adjustable on a physiological and physical level, and based on an understanding of the variables that interfere with oximetry signals, to optimally minimize specific interferences with the oximetry signal.